Chorus II mods

[g3dahl]

Hi djk,

Thanks for the informative post! It is certainly interesting to see how the Altec products evolved over the years.

I had indeed noticed that the sensitivity specs I had quoted were taken at 4' rather than 1m. They came from the 311-60/90 data sheet, which lists sensitivity at 4' for various combinations of 311's and drivers. I just looked again...with the 290-4K driver, it says 109.0 dB in the 311-60 and 106.5 dB in the 311-90. For all the other drivers the relationship is the same: +2.5 dB in the 311-60 over the spec for the 311-90. This is consistent with theory, because the energy is contained within a smaller dispersion pattern.

In any event, I'm sure I will need to attenuate my 290-8K's, and expect that the Universal autoformers will provide enough flexibility to get a good match with the woofer.

Q-man has reported getting 3 dB less sensitivity with the 16-ohm diaphragms (compared with the 8-ohm), and I have been trying to reconcile that with the Altec sensitivity specs, which don't vary with impedance. Perhaps the autoformer in the Klipsch-style crossover accounts for Q-man's different results.

By the way, are you the same djk who has posted on the Audio Asylum's High Efficiency Speaker forum? If so, I was reading some of your old posts last night. The subject involved using a simple resistor to attenuate the 290, resulting in enhanced HF extension. I have recently become concerned about whether the 290's will reach high enough for the tweeters I had planned to use, but I was expecting to use autoformer attenuation, for the same reasons PWK and ALK have used them. Any thoughts or guidelines? Thanks!

Gary Dahl

[g3dahl]

Another twist here.

The poor MLSSA showing by the stock Klipsch tweeter was due, at least in part, to a diaphragm compatibility issue.

The night before I brough one of the speakers to Lynn's house, one of the tweeters stopped working. After verifying that its diaphragm was open, I installed a replacement diaphragm. While doing so, I noticed that the original was phenolic, but the replacement was not. The material looked like the typical black cloth that one finds on typical dome tweeters. Otherwise they appeared to be the same, but the new one didn't seem to fit as perfectly as the old. I should have paid more attention to that fact.

After "retiring" the Klipsch K-79-K tweeters in favor of the Fostex FT17H's, I spent some time trying to repair the open-circuit phenolic diaphragm...what did I have to lose? It turned out that the ultra-fine wire leading from one terminal to the voice coil (it appears to all be one piece of wire) had broken next to the terminal. After a few unsuccessful tries, I finally got it spliced back together. I'll be anxious to try the phenolics once again.

But when I was removing the non-phenolic replacement diaphragm, I became quite aware of the fit problem--the replacement voice coil was (slightly) binding on the inner surface of the magnetic gap! No wonder the thing was 10 dB down and had no extension! The other one was exactly the same.

I called the Klipsch Parts Department. They sell the same replacement diaphragms (#127103) for all versions of the Chorus. The rep didn't know anything about diaphragm materials...but he did say that they also sell the entire driver assembly (#129102). At just $40 per piece, they might be worth a try. For that matter, maybe I should look into the later-production mid driver as well.

Gary Dahl

[Q-Man]

Gary and djk are right about the magnets in the Altec drivers.

I had a discussion with Bill at Great Plains Audio a while back and we were talking about the 290-16K drivers when he said that they didn't have Alnico magnets. I didn't even think about the older C,D,E,G,and H 290's when I said that. So I stand corrected.

I have three 290-16K drivers and two 290-16L drivers. They all have the tangerine phase plug. djk said that the ferrite 290's all had the tangerine phase plug so it seems like the ones that you are waiting for will have them.

I had a problem with one of my drivers being about 3dB louder then the others. I again called Bill at Great Plains Audio to find out what might be the problem. The first thing that he had me check was the diaphragm. Bill was the one that told me that an 8 ohm diaphragm will be about 3dB louder than a 16 ohm one. I also found this to be true with a pair of 288-16K drivers that I got off of ebay. One was louder then the other and it was corrected by putting a 16 ohm diaphragm in it.

[Q-Man]

Gary,

Where do you plan, or have to crossover to the tweeter? As you said, the 290 K series is rated to 7kHz. I crossover at around 6kHz and there isn't any dip in it's response at that point.

Bill also said that with the little bit of power we are going to be feeding these drivers that you can push them more towards their extreme frequency range limits. They can run all day long with 100 to 120 continuous watts being fed to them. They will never see anything close to that from your network.

[g3dahl]

Hi Q-man,

If I can cross over around 6 kHz or so, that will be great. Someone on Audio Asylum had reported that his 290's started dropping 6 dB/oct at about 2 kHz, and were 10 dB down by 7 kHz. I don't know which version it was. But none of the tweeters I was considering were usable down to 2 kHz!

Right now I am using the Fostex FT17H tweeters, with the stock Chorus II crossover (about 5-6 kHz). Their recommended crossover frequency is "more than 5 kHz" according to the spec sheet. They sound to me like they would be happier with a somewhat higher crossover, or perhaps a steeper slope. I am also considering other tweeters, including the later-version K-79-K, which is said to outperform the earlier units.

The 290's and the Universal autoformers arrived this evening, so new fun experiments will soon be underway.

Thanks for all the helpful information and comments!

Gary Dahl

[djk]

"Q-man has reported getting 3 dB less sensitivity with the 16-ohm diaphragms (compared with the 8-ohm), and I have been trying to reconcile that with the Altec sensitivity specs, which don't vary with impedance. Perhaps the autoformer in the Klipsch-style crossover accounts for Q-man's different results."

The efficency is the same for all impedance types, 108.2dB/W/1M for the big motor jobs on the 311-90 horn. But remember 1W is a different voltage depending on 4-8-16 ohms. With the driver matched to the correct taps on a T2A the level and the cap values all work out. 16R goes to tap 4, 8R goes to tap 3, 4R goes to tap 2, all will play the same volume level and have the same crossover frequency without changing the cap.

"By the way, are you the same djk who has posted on the Audio Asylum's High Efficiency Speaker forum?"

Yes

"If so, I was reading some of your old posts last night. The subject involved using a simple resistor to attenuate the 290, resulting in enhanced HF extension. I have recently become concerned about whether the 290's will reach high enough for the tweeters I had planned to use, but I was expecting to use autoformer attenuation, for the same reasons PWK and ALK have used them. Any thoughts or guidelines? Thanks!"

The HF response is a combination of both the horn and the driver. Q-man says his make it to 6Khz on the 311-90, they need some help to do that on the little $25 horn from MCM that I use. Because of the high voice coil inductance a simple series resistor will actually boost the highs while cutting the mids, I got ±1dB from like 600hz ~6Khz with my 290-4C with a big resistor in series.

Alternately just move it down a tap until the midrange is right. Then add a filter from the input to the output. You will have between 9dB~12dB of attenuation to selectively remove to boost the high end near your tweeter crossover point. Shouldn't be any problem getting enough output. Note: The 4R and 8R drivers need more attenuation, therefore they have more energy available for this application.

"If I can cross over around 6 kHz or so, that will be great. Someone on Audio Asylum had reported that his 290's started dropping 6 dB/oct at about 2 kHz, and were 10 dB down by 7 kHz. I don't know which version it was."

They all roll-off above 2.5Khz or so. It depends on what your horn is doing at that point. If you put a 290 on a constant directivity horn it will be 10dB down at 7Khz. You will just have to wait and see how yours measure, then 'fix' it in the network.

[g3dahl]

Thanks! Great info!!

Gary Dahl

[sunnysal]

Gary, all this attention lavished on the "wrong" speakers! LOL! I just need to convince you and Lynn to turn your attention to the "real" Klipsch masterpiece, the Klipschorn. Then I will sit back, watch and copy your moves, as I am certain all the chorus owners are doing now. what a waste! LOL! Maybe some of these lessons are applicable to my speakers. tony

[LynnOlson]

Might be a while 'till we get to the Khorns, since I don't think Gary or I are planning to buy a new house any time soon. Belle Klipsch or La Scala, yeah, a possibility there.

One thing I have to mention is that MLSSA shows all horn systems in an unflattering light. What horns do best is very low IM distortion, and this is difficult to measure with MLSSA, or any system, without an anechoic chamber (big bucks there). MLSSA favors speakers with good time domain performance, and the champs here are electrostats and a few direct-radiators. By comparison, horn speakers don't come off well when you look at MLSSA data - this measurement bias is reflected in the high-end biz in general.

So why I am messing around with MLSSA and Chorus at all? Well, I've owned my own MLSSA system since 1991, so I know how to drive the thing (working at Tektronix Spectrum Analyzers and before that, Audionics, helped in this respect). Even though it doesn't conveniently measure IM distortion, I can use it to optimize the time and frequency performance of a horn speaker, keeping in mind that decreasing crossover slopes exacts a price in higher distortion (true with any drivers, not just horns).

It's also a little zany and offbeat to apply BBC/KEF/Quad philosophy to a horn speaker, which is usually considered the polar opposite in the speaker-design community. But why not? No reason to keep the sonic balance of the mid-Forties Altec Duplex around forever; it was state of the art then, but we can measure things better today, and can do better with modern crossovers.

I have to admit an all-horn system is a lot bigger challenge than the Cornwall/Chorus/Forte series. Horns don't like being "stretched" out of their natural bandwidths (lots of lumps, lots of distortion), and the region where the bass horn quits (300Hz or so) and where the mid horn comes looks tough. Neither horn is really comfortable around 400 to 500 Hz, and that's a critical region of the spectrum. It's the center of the musical power band (most energy), the region where voices come in (bottom of telephone bandwidth), and the few studies that indicate that phase is audible indicate that 500Hz is where it is most audible. The room is starting to get goofy at 300Hz and below, which doesn't help. In fact, unless you have a *really big* anechoic chamber, most measurement techniques require you to switch to nearfield below 300Hz since room effects start to dominate the measurements.

If you're getting the idea that 200 to 500 Hz is a rough region for a crossover - much less horn crossovers, which don't "stretch" in any meaningful way - you're right. The Cornwall/Chorus/Forte make all of this a lot easier since getting a 15" or 12" woofer up to 1kHz is no big trick. You filter off the midrange bump (if any) and you're all done. Instant 700Hz crossover.

But the Khorn/Belle/LaScala are another story altogether - getting the bass and mid horns on speaking terms is not a simple matter. It can be brute-forced by going with a *big* Altec theatre horn, or an even more gigantic Edgar salad-bowl horn. These are all big, room-dominating horns, looming over the top of a Khorn. Al K has a more elegant approach with an ultra-steep crossover, maybe even an elliptic filter, in order to keep the horns well-behaved on the edge of their usable range. But no free lunch there either; elliptic filters have substantial phase distortion, but then again, the several-millisecond path-length difference between bass and mid horn probably renders phase distortion moot.

The waters get really deep with an all-horn system, mostly around the bass-mid transition. The Chorus lets me take little baby steps with horn systems, letting me find out what horns like to do and what they don't like to do. I think my next experiments will be with the Beyma tweeters - the mid-tweeter crossover is a lot simpler.

[g3dahl]

Hi djk,

To make sure I follow your meaning:

1) The "simple series resistor" implies that you don't also have a shunt resistor, right? Actually, I was attracted to ALK's implementation, where there is a shunt resistor across the autoformer which reduces the driver's influence on the upstream circuitry. Are these topologies mutually exclusive?

2) "Add a filter from the input to the output" -- do you mean a capacitor between the top of the autoformer and the output tap being used, to create a shelf filter?

I can tell already that I need to put together an external crossover for this project, so I can make changes easily without pulling the terminal box off the back of the speaker for every revision!

Oh, one more question: When disassembling one of the 290's, I noticed the internal metal cover that provides a mounting surface for the optional 70-volt transformer. My 288's don't have any sort of hard surface this close to the diaphragm--just a felt pad on the inside of the rear cover. Do you use the internal cover, or have you experimented with alternatives? I have heard that some people even prefer to leave the back covers off! I was considering removing the internal cover and installing a pad inside the rear cover, or perhaps using the 288 rear covers instead.

Gary Dahl

[djk]

"1) The "simple series resistor" implies that you don't also have a shunt resistor, right? Actually, I was attracted to ALK's implementation, where there is a shunt resistor across the autoformer which reduces the driver's influence on the upstream circuitry. Are these topologies mutually exclusive?"

I got a flatter response and it sounded better with just a series resistor, no shunt. I use whatever works, no point in getting hung up on theory, especially if it doesn't work. Theory is the place to start. Then you start hanging parts in there and measuring it.

2) "Add a filter from the input to the output" -- do you mean a capacitor between the top of the autoformer and the output tap being used, to create a shelf filter?

Yes, could be a cap could be a cap with a resistor in series or parallel.

If you use a traditional L-pad you may need a LCR tank circuit in parallel with the series arm of the L-pad to get enough midrange in the 6Khz range.

"I can tell already that I need to put together an external crossover for this project, so I can make changes easily without pulling the terminal box off the back of the speaker for every revision!"

I always use the card-table-next-to-speaker method of crossover design.

"Oh, one more question: When disassembling one of the 290's, I noticed the internal metal cover that provides a mounting surface for the optional 70-volt transformer. My 288's don't have any sort of hard surface this close to the diaphragm--just a felt pad on the inside of the rear cover. Do you use the internal cover, or have you experimented with alternatives? I have heard that some people even prefer to leave the back covers off! I was considering removing the internal cover and installing a pad inside the rear cover, or perhaps using the 288 rear covers instead."

I usually remove the big back cover on the 290 and just run the internal cover that the transformer mounts to. The 292 has the same back cover the 288 does.

[sfogg]

Lynn,

"Belle Klipsch or La Scala, yeah, a possibility there."

As a new La Scala owner I'd love to see what you would do with a pair of them. With the same driver and horn (not including bass horn) of the K'Horn I'd think some of the work would carry over there. If you were near me (Ma.) I'd loan you a pair of La Scala's with ALKs, AA or AL crossovers in them.

Shawn

[g3dahl]

Well, we didn't have very much luck with the 290's. We tried all of the tricks we could think of to extend their HF response to a usable point, but no luck. The 311-60's were probably a factor.

So, we moved on to the 288 driver. HF extension was of course much better, and after some messing around we ended up with a beautiful response curve, remarkable in fact! The finished configuration was a 2-way, with the 311-60's running all the way up. The available tweeters didn't have significant output beyond the HF limits of the 288 anyway, a bit short of 15 kHz.

Initial listening tests at my house were extremely thrilling. The Patricia Barber SACD was un-friggin'-believeable! Later in the day, however, I put on some orchestral music, which is my most important test, because I am an orchestral conductor. Bad news. Upper strings were a mess. The <15kHz HF extension limit wasn't the problem, but rather the resolution and distortion level of the treble that was there. The same system that was hair-raisingly awesome with blues/jazz was unacceptable (to my ears) with orchestral strings. Hmmm.

I think I was closer to what I was looking for with the Chorus II/Fostex tweeter, using the Klipsch squawker, rather than the Altec 311-60/288 combo, which was a huge surprise. The Universal 3619 autoformers are now here, which would have allowed us to dial-in the Klipsch squawker without the series resistor. I'm now thinking that the next step is to go back to the Klipsch squawker with the Universal autoformer, and to find a better tweeter. Lynn is looking seriously at the Beyma CF25, and I will too. I am also thinking of trying the later version of the K-79-K...mine had phenolic diaphragms, but the last ones used polymers.

Gary Dahl

[sunnysal]

Gary,

surprising results...sounds to me that you are on the right rack there at the end of the post. back to klipsch squawker and perhaps the polymer klipsch tweeter, pop in the autoformer, play with levels and see what ya get. maybe klipsch really did some work on voicing the speakers after all?!. keep us informed. tony

[g3dahl]

Lynn and I got together for another round of Chorus mods. This time, the goal was to see how far we could go with the stock woofer and squawker, but using the Universal 3619 autoformer. I brought the original Klipsch tweeters as well as the Fostex FT17H's.

We started with the Klipsch tweeter, and the autoformer set on tap 2, which is one step lower than it would have been with the original autoformer. The tweeter crossover used stock values; the woofer and squawker were set the way they had been at the end of the Altec experiments.

We had lots of trouble with the Klipsch tweeters because the joint between the terminals and the voice coil wire is easily broken. I was able to re-melt and get them working again a few times but eventually they couldn't be revived. Better to mount these tweeters and leave them be, rather than subject them to what I did!

Anyway, we had already optimized the crossover while we still had a working Klipsch tweeter, and I have more diaphragms on the way. So, we turned our attention to the Fostex tweeter. By this time, we were experimenting with a time-aligned configuration, with the tweeter on top of the speaker enclosure, set about a foot back from the front panel. Once everything was dialed in, we had a remarkably smooth and flat response, far superior to our previous efforts. Now I will have to build the changes into the "permanent" (Ha!) crossovers so we can start listening to something more interesting than MLSSA test signals!

Lynn will be posting the MLSSA plots when they are ready, and perhaps the schematic. The topology is mostly similar to the original, but most of the cap values have changed, and resistors are used differently.

Gary Dahl

[sunnysal]

after moving the tweeter back how to you plan to keep the reflections off the top of the cab from muddying the sound? when you have a chance, please post the schematic for the upgraded x-over I would be interested to see what values you ended up with compared with stock. I am dying to hear what you guys think of the sound once you get a chance to sit down to some music, I forget if you kept a stock speaker around for comparison purposes. (if not a b-boarder may be willing to bring a similar model over for a "comparo" shoot out) regards, tony

[g3dahl]

For the purpose of measurements, we placed the tweeter on top of a couple of thick books (Harry Potter #5 and The Adventures of Sherlock Holmes) and used a folded-up bath towel to cover the rest of the enclosure's top surface. Obviously this worked, because the impulse response measurement results came out very clean. Eventually, I will build a wooden enclosure for the tweeter, and try to come up with a sound-absorbent treatment for the cabinet top that is reasonably attractive.

There were quite a few changes to the crossover. Going from memory:

The second tweeter cap (2 uF) has an extra 4.7 uF in parallel, and there is now a 50 ohm resistor in parallel with the Fostex FT17H tweeter. The polarity is reversed, as in the original.

On the squawker, the Universal 3619 autotransformer has a 10 ohm resistor in parallel (a la ALK). Output is taken from tap #1. Squawker polarity is now reversed.

For the woofer section, the capacitor has been changed from 68 uF to 20 uF with a 3-ohm resistor in series. BTW, the 3-ohm value turned out to be critical.

The inductors are stock, as is the first tweeter cap (also 2 uF) and the squawker cap (6 uF), though all caps have been replaced with premium-quality polypropylene films.

This crossover is for the time-aligned version with the Fostex FT17H tweeter on top of the cabinet only! With the stock tweeter in its normal location, best results were obtained with different parts values...but the response was not nearly as smooth as our final version using the Fostex tweeter. Also, the Fostex/time-aligned version has much cleaner impulse response.

I will post again when I have had an opportunity to listen to music!

Gary Dahl

[sunnysal]

thanks for the info. I look forward to hearing the results of the listening tests...then I will just have to figure out how to translate this work into something to guide me with modding my k-horns...LOL tony

[g3dahl]

One thing I didn't mention when describing the crossover circuit: the 20 ohm series resistor has been removed from the squawker.

Gary Dahl

[LynnOlson]

Hi all, sorry for the late posting, but things have been busy around here, with a new HVAC system installed today. The twenty-year-old Carrier heat pump that came with the house quit (gee, these things don't last forever?) and finally got the new one installed and running today. The weather here in the Puget Sound region is usually mild, but it has a way of throwing curves at us every now and then.

Had another measuring and modifyin session yesterday (I'd like to name this thread Chorus II Part Deux) and liked what we've done so far. Time-aligning the tweeter really chases out those annoying hill-and-dale ripples that seem to plague horn designs. Usually, you twiddle with the phase response (altering crossover slopes, reversing driver phase, etc.) but that seems to just chase the nulls from one part of the spectrum to another. When a design is not responding to the usual crossover tweaks, it's trying to tell you something ... to stop messing with the crossover, and look a little deeper at what's going on.

In this case, the mid and tweeter are misaligned by about 1mSec. This is obvious in the time response of the earlier measurements, where you see two peaks, one from the mid, and the one from the tweeter. A millisecond doesn't sound like much, but it's five wavelengths at 5kHz. Looking at it that way, it's a lot.

It's a truism in speaker design you can't correct a time error with phase adjustments. Phase is relative to one frequency only, usually the nominal crossover frequency. This implies a phase adjustment that's right for the crossover will be way out an octave above and below. And that's just what we saw on MLSSA - nulls and peaks that moved around, but never going away.

Since I'm used to working with direct-radiators, this was a new one for me, ripples that just refused to go away. But I thought a bit about the severe, many-wavelength misalignment of the mid and HF driver. It's not pretty in the time domain - two spikes for the price of one - and has to be having bad effects on the FFT, which is really allergic to reflections of any kind. Even a heavily damped floor reflection at 3mSec (with 20dB of attenuation from many pillows) still creates small ripples in the FFT, so what can two big peaks 1mSec apart be doing?

A lot, as it turns out. Once Gary and I moved the tweeter towards the rear of the enclosure (centerline of the tweeter about 6 inches above the cabinet top with several layers of towels on the cabinet top in front of the tweeter), the crossover pretty much dropped into place. The two spikes in the time display merged into one, and the crossover started to behave normally, with the crossover region nice and smooth for a change. In fact, the entire response of the Chorus/Fostex fits into a 5dB window - that's right, plus or minus 2.5dB, with no smoothing at all.

MLSSA data to follow, stay tuned.